1. Field of the Iinvention
This invention relates to a power control device controlling supply of electric power to a load by making use of electric power due to charge and discharge of an electric power storage device as well as electric power from a power supply.
2. Description of the Related Art
Power control devices of the above-described type include an electric power storage device so that a predetermined amount of electric power is supplied to a load even under occurrence of load variations and/or power supply variations. Electric energy stored in the power storage device is used for supply of power to the load. For example, an unterruptable power source comprises an AC/DC converter converting AC power to the corresponding DC power, a DC/AC inverter converting the DC power delivered from the converter to the corresponding AC power which is supplied to a load, and an electric power storage device connected between the converter and inverter. An AC input is converted by the AC/DC converter to the corresponding DC output under a normal condition. The DC output is converted by the inverter to an AC output having a desired current, voltage and frequency, the AC output being supplied to the load. The aforesaid DC output is partly used for charge to the power storage device, whereupon electric energy is stored in the power storage device. The electric energy stored in the power storage device is supplied to the load side upon occurrence of power failure. As a result, the load can be prevented from an interrupt.
The aforesaid power storage device comprises a secondary battery with a relatively long discharge duration, for example, a lead-acid battery, lithium ion battery or sodium ion battery (NaS battery). Alternatively, a multitude of aluminum solid electrolytic capacitors connected to one another are used as the power storage device. The above-described conventional power control device has the following problems. Firstly, each of the aforesaid power storage devices has a short cycle life and is accordingly handled as an expendable component. In the case of a lead-acid battery, for example, when the electric charge and discharge are alternately repeated at 200 to 1000 times under the use at normal depths of charge and discharge, deterioration of electrodes etc. results in a power reduction, whereupon the efficiency of the power control device is reduced. In view of the fact, the lead-acid battery needs to be replaced by a new one at the intervals of 2 or 3 years. Thus, the maintenance of power storage device necessitates much labor and large cost.
Secondly, the secondary batteries used as the power storage device contains materials detrimental to environment, for example, lead, acid, sulfur and lithium. Accordingly, sufficient maintenance is required in order that breakage of the power storage device may not result in environmental destruction. This disadvantageously increases the maintenance cost. Further, since the power storage device containing detrimental materials needs to be replaced by a new one at the intervals of 2 or 3 years, disposal of the power storage device requires a special treatment facility, resulting in high costs for treatment.
Therefore, an object of the present invention is to provide a power control device which has a high reliability, is easy to maintain and can reduce costs for environmental countermeasures.
The present invention provides a power control device comprising an electric power storage device provided across a power supply line for a load and including at least one electric double layer capacitor (EDLC) bank and a secondary battery combined with the EDLC bank, the EDLC bank including a plurality of parallel-connected rows of EDLC unit cells, each of the rows including a plurality of series-connected EDLC unit cells, and a control device controlling the electric power storage device so that when an input power to the electric power storage device is interrupted so as to result in a power interrupt, the EDLC bank supplies electric power to the load for an initial period of the power interrupt, and thereafter the secondary battery supplies electric power to the load.
Voltage and capacity both suitable for power control can be obtained since the electric double layer capacitor (EDLC) bank includes a plurality of EDLC unit cells configured into a series-parallel connection. Electric power is stored in the EDLC bank when a power supply has a sufficient amount of power to be supplied. The power stored in the EDLC bank is consumed when an amount of power consumed by the load exceeds a power-supplying capacity of the bank due to power stoppage, power supply variations or load variations. Consequently, a stable amount of electric power can be supplied to the load.
The EDLC bank can perform high-speed electric charge and discharge and accordingly has a high charging efficiency. As a result, power to be regenerated in a short time can efficiently be recovered by the EDLC bank and accordingly, the efficiency of the power control device can be improved. Furthermore, since the EDLC bank has a high output density, a large power can be supplied and received between the power supply and the load within a short time. Consequently, a stable high-quality power can be supplied to the load. Additionally, the EDLC bank contains no material contaminating environment, and a cycle life of the EDLC bank is as long as or longer than a service life of the power control device. Thus, since the EDLC bank need not be used as an expendable component and replaced by a new one, the maintenance of the power control device can be simplified and costs for measures to protect environment can be reduced.
Furthermore, the electric power storage device includes at least one EDLC bank and a secondary battery combined with the EDLC bank. Consequently, a high energy density of the secondary battery can be obtained as well as the high-speed charging and discharging performances, high charging efficiency and high output density of the EDLC bank. For example, a regenerative power obtained from the load in a short time is stored in the EDLC bank, whereas a regenerative power gradually obtained from the load in a long time is stored in the secondary battery. Further, provision of the secondary battery can realize power backup for a long time. Accordingly, supply of power to the load can be rendered possible for a long time even in power stoppage.
Still further, the electric power storage device includes at least one EDLC bank and an aluminum solid electrolytic capacitor combined with the EDLC bank. An aluminum solid electrolytic capacitor can absorb switching ripple current from the converter and the inverter. Further, a power variation in which frequency twice as high as that of a fundamental wave occurs at the DC side in an arrangement of compensating unbalance in a power system to which the inverter is connected or of converting the DC power from the electric power storage device to an AC power which is supplied to an unbalanced load. The aluminum solid electrolytic capacitor can also absorb the aforesaid power variation.
By making use of power stored in the aluminum solid electrolytic capacitor, a desired power can be supplied to the load in a very short period (several tens msec. or less) at an initial stage of sudden power supply variations such as power stoppage. In a subsequent period, power stored in the EDLC bank is supplied to the load. Thus, a stable power can be supplied to the load for a long period of time immediately after the sudden power supply variations such as power stoppage.
Yet still further, the electric power storage device includes at least one EDLC bank, an aluminum solid electrolytic capacitor and a secondary battery, the latter two of which are combined with the EDLC bank. Consequently, the above-described effects can be achieved simultaneously.
Furthermore, each EDLC unit cell has an internal resistance which is at or below 2 mxcexa9 and a product of an electrostatic capacity of each EDLC unit cell by the internal resistance thereof is at or below 4 xcexa9F. The arrangement is suitable for a case where variations at intervals of several hours or less in the load or power supply is leveled by energy stored in the EDLC. For example, electric power is supplied to a load varying at an interval shorter than several hours so that the load is leveled, whereupon influences on the power system can be rendered smaller. In this usage in which the power supply variations are compensated, power input to and output from the EDLC bank are frequent. As a result, loss of power due to an internal resistance tends to be increased. In accordance with the fourth preferred form, however, the internal resistance of each EDLC unit cell is set at a small value. Further, the internal resistance of each cell also depends upon the electrostatic capacity thereof. Accordingly, the product of the electrostatic capacity by the internal resistance serves as a value evaluating the internal resistance in the relationship with the electrostatic capacity and is set at a small value. Consequently, power loss can be reduced in the EDLC bank and the efficiency can be improved.
Yet still further, when the EDLC bank is used for a primary purpose of electric power storage, each EDLC unit cell has an internal resistance which is at or below 10 mxcexa9 and a product of an electrostatic capacity of each EDLC unit cell by the internal resistance thereof is at or below 100 xcexa9F. This arrangement is suitable for an uninterruptable power control unit which has a primary purpose of electric power storage while storing power for a relatively long time. More specifically, the arrangement is suitable for power control at intervals longer than several hours and equal to or shorter than several days. For example, electric power is supplied to a load varying at an interval of several days by the above-described arrangement so that the load is leveled, whereupon the power system can be operated efficiently.
Power loss due to the internal resistance of the EDLC bank is easy to reduce since the frequency of power input and output to and from the EDLC bank is low in the aforesaid power control. Accordingly, the internal resistance and the product of the electrostatic capacity by the internal resistance can be set at respective larger values as compared with a case where power control is performed under a load or power supply varying at intervals of several hours or less. Consequently, the internal resistance of each EDLC unit cell is increased such that the electrostatic capacity can be increased, and accordingly, a larger capacity of electric power can be stored.
Furthermore, a condition expressed by Y greater than 100xc3x97Xxe2x88x920.8 is met where Y designates an energy density of each EDLC unit cell in Wh/kg and X designates an output density of each EDLC unit cell in W/kg. The inventors inspected the Ragone plot indicative of the relationship between energy density (Wh/kg) and output density (W/kg). The inventors then found the above-described condition from the results of the inspection. When the used EDLC unit cells meet the condition, the efficiency and performance in the power control by the EDLC bank can be rendered maximum.
Yet further, the electric power storage device includes at least one secondary battery having an energy density which is at or above 10 Wh/kg. This arrangement accomplishes an electric power storage device with a high energy density which cannot be achieved by the EDLC bank alone. Consequently, since an amount of energy stored is increased, electric power can be supplied to the load for a longer time.
Still yet further, the electric power storage device includes at least one aluminum solid electrolytic capacitor having an output density which is at or above 10,000 W/kg. This arrangement accomplishes an electric power storage device with a high output density which cannot be achieved by the EDLC bank alone. Consequently, the arrangement is suitable for a case where input and output of a large power are each performed in a short period of time.